customized constructors

[selvakumar Thiyagarajan]

what would i need to do if only one instance of a class should be produced?
anyone can use only one object..(Question asked in HF java book)

soln:
This is sample class code snippet.Please say whether it is correct for the above question...
public class firstclass
{
static count=0;

firstclass()
{
if (count<=0){
system.out.println("object created");
count++;
}

}
}

[Ankit Garg]

Hi selvakumar ! I think the question is about singleton pattern. The solution would look like this

class Singleton
{
    private Singleton(){}     //now no external class can create instance of Singleton class
    private static Singleton obj;
    public static Singleton getInstance()
    {
        if(obj == null)
        {
            obj = new Singleton();
        }
        return obj;
    }
}

[Maneesh Godbole]

It might be a good idea to to make the getInstance() synchronized.

[Campbell Ritchie]

Or you have a private static Singleton instance = new Singleton(); field. Then you can return instance from the getInstance() method.

[Ankit Garg]

Campbell Ritchie wrote:Or you have a private static Singleton instance = new Singleton(); field. Then you can return instance from the getInstance() method.

There's a reason to why I didn't write that. Once I wrote that and someone got confused and said that the code must result in a StackOverflowError. He was confused about instantiating instance fields of a class in itself . If someone is confused in which case the StackOverflowError will occur, this program will generate that error

class Erroneous
{
    Erroneous obj = new Erroneous();
} 

[Campbell Ritchie]

It was whoever told you off who was in error; as you will see, you only get a stack overflow if the instance field is not static.

[Ankit Garg]

Campbell Ritchie wrote:It was whoever told you off who was in error; as you will see, you only get a stack overflow if the instance field is not static.

Yes I know that. I was just saying that I didn't wrote the program with direct instantiation of the static field because it tends to confuse some people. What you are saying is 100% correct and I agree with you ...

[Campbell Ritchie]

You will have to educate your colleague; that is one of the ways to implement a Singleton recommended by Joshua Bloch in Effective Java 2/e page 17-18. As I said, it is the other person who is mistaken about this.

[Ankit Garg]

Campbell Ritchie wrote:As I said, it is the other person who is mistaken about this.

Ohh, I thought that you are thinking that I am confused with that. Actually I was confused about what you said about people being confused because I thought that you are thinking that I am confused (now that's a confusing statement )

[Campbell Ritchie]

And are you more confused or less confused about it now?